1. Field of the Invention
The invention disclosed herein relates to a semiconductor memory device using a semiconductor element and a method for driving the semiconductor memory device.
2. Description of the Related Art
At present, a dominating non-volatile ROM is a batch-erasing-type memory referred to as a flash memory. A flash memory is largely divided into a NOR type flash memory and a NAND type flash memory. A NOR type flash memory is used for firmware, a BIOS, and program code storage of an electronic appliance such as a mobile phone. A NAND type flash memory is used for a large capacity memory and an SD card.
Although a NOR type memory device has high reliability, an increase in speed at the time of writing, an improvement in power consumption, an improvement in integration degree, and the like are issues of a NOR type memory device. Although a NAND type memory device can have a high integration degree and large capacity, and a writing speed of the NAND type memory device is faster than that of a NOR type memory device, an improvement in reliability, a development in a function for correcting an error, and the like are issues of the NAND type memory device.
A flash memory includes a storage MOS transistor which includes a minute floating gate surrounded by an insulating film, a wiring for data input/output, and the like, and charge is accumulated in the floating gate to hold data. In addition, data is rewritten by compulsory flow of charge in a thin oxide film in contact with the floating gate. Therefore, the degradation of the film cannot be ignored. Further, there is a limitation on the number of writings and the lifetime is short. In addition, it has been impossible to concurrently perform writing operation and reading operation (for example, see Patent Document 1).
In addition, the cost of a semiconductor memory device generally depends on the size of the semiconductor memory device. Development for achieving high integration and reducing cost has been conducted by reducing an area occupied by a semiconductor element or a driving circuit in a semiconductor memory device. As a technique for miniaturizing a semiconductor element, a technique relating to a next-generation semiconductor referred to as a three-dimensional semiconductor has also been developed.
In view of high integration of a semiconductor memory device, attention has been drawn to development of a multi-valued memory technique. In a multi-valued memory in which data having any one of values more than two values (one bit) is stored in one memory cell, storage capacitance can be greatly increased as compared to a binary memory. However, in order to detect 2n storage states in a multi-valued memory, threshold voltages of the group of memory cells need to be set to 2n−1 levels or more, and in addition, a plurality of detection cycles on the basis of the respective different threshold levels are also needed. For now, a multi-valued memory of eight values (3 bits) or more is not practically used.